1. Field of the Invention
The invention relates to a method of preparing terephthalic dialdehyde and isophthalic dialdehyde.
2. Discussion of Prior Art
For the sake of simplicity the following abbreviations will be used herein:
p-100=p-methylbenzyl chloride* PA1 p-200=p-methylbenzal chloride** PA1 p-101=1,4-bis-(chloromethyl)benzene PA1 p-201=1-dichloromethyl-4-chloromethylbenzene PA1 p-202=1,4-bis-(dichloromethyl)benzene PA1 p-302=1-trichloromethyl-4-dichloromethylbenzene. FNT *or p-chloromethyl benzene **or p-dichloromethyl benzene
The preparation of a number of aldehydes can be performed through a reaction of the corresponding --CH.sub.2 Cl compounds with hexamethylenetetramine, while other aldehydes obviously cannot be obtained in this manner.
One possibility for the preparation of the terephthalic aldehyde consists in the reaction of pure 1,4-bis-(chloromethyl)benzene with hexamethylenetetramine, but it results in yields of only 34% of the theory.
This method has a number of additional disadvantages: The side-chain chlorination of p-xylene is not selective, and instead the various possible chlorination stages are obtained together. The ratio of the individual chlorination products depends exclusively on the degree of chlorination. The amount of 1,4-bis-(chloromethyl)benzene passes during chlorination through a maximum which occurs at 46 wt-% Cl and a chlorination degree of 2.4. The largest percentage of p-101 is therefore present when the degree of chlorination* has already far exceeded a value of 2.0 for pure p-101. Upon further chlorination, 1,4-bis-(chloromethyl)benzene decreases again in favor of the formation of, for example, 1,4-bis-(dichloromethyl)benzene and more highly chlorinated xylenes. FNT *the degree of chlorination being befined as the number of chlorine atoms in the side chains of a molecule of a substance or a mixture thereof.
Accordingly, the separation of 1,4-bis-(chloromethyl)benzene from the six-component mixture is difficult, inasmuch as the boiling points are close together.